1. Field of the Invention
The present invention relates to a light measuring device, and more particularly to a light measuring device for an exposure control device.
2. Description of the Related Art
Heretofore, as a light measuring device for a single lens reflex camera, a device which leads a bundle of light to a photoelectric conversion part by such a beam splitting member as for example, a micro beam splitter formed at a condenser lens of a viewfinder optical system, or a device which leads the bundle of light to a photoelectric conversion part through a secondary imaging lens provided at a bundle of light exit plane of a pentagonal prism, etc., have been proposed.
However, devices mentioned above have problems. For example, many steps are necessary for adjusting the sensitivity of the distribution of the quantity of light at a field of view of a viewfinder and for adjusting the distribution of light measuring sensitivity of a split image prism on a focusing screen.
FIGS. 5, 6 and 7 are cross-sectional views for a single lens reflex camera showing an example of prior art, wherein the same components are identified with the same numbers.
In FIG. 5, the light beam from an object (the optical axis of which is shown by one dot chain line 2) passing through a taking lens 1 is reflected upwards by a mirror 3 and comes to an eye piece lens 8 through a focusing screen 4, a beam splitting member 5, a condenser lens 6 and a pentagonal prism 7, while the bundle of light split by the beam splitting member 5 impinges on a photoelectric conversion part 9. 10 denotes a film plane. In this system when an object is viewed by the eye piece lens 8 a portion of light beam reaching the eye piece lens 8 appears to be missing because of the splitting of the light beam by the beam splitting member 5, thereby generating a shadow with a clear contour at a central part of a viewfinder.
In FIG. 6, the light beam from the object penetrating the taking lens 1 is reflected by the mirror 3 and proceeds through the focusing screen 4, then has its direction changed by the pentagonal prism 7 and reaches the eye piece lens 8. Also, the central part of the focusing screen 4 provides an image to the photoelectric conversion part 9 by a secondary imaging lens 11 provided at the light exit plane of the pentagonal prism 7. In this system, a shadow will not be generated at the central part of the viewfinder, being different from the example shown in FIG. 5, but many steps of work will be needed for adjusting the means for forming an image of the central part of the focusing screen 4 at a center of the photoelectric conversion part 9. Further, since the bundle of light after passing through the focusing screen 4 is measured, there is the shortcoming that the distribution of light measuring sensitivity is influenced by the split image prism 4', etc. of the focusing screen 4; also there is the problem that the focusing screen 4 can not be freely exchanged.
Also, FIG. 7 shows an example in which a photoelectric conversion part 12 of a light measuring device and a photoelectric conversion part 13 of a focus detection device are respectively positioned at a bottom part of a mirror box, wherein a half-reflection part 15 is formed at a central part of a main mirror 14 and a beam splitting mirror 16 is provided at a rear part of the same. The beam splitting mirror 16 has a half-reflection plane 17 formed at an object side and a total-reflection plane 18 formed at the film plane 10 side thereof. The bundle of light from an object is, after passing through the taking lens 1, split into two directions by the main mirror 14 and one of the thus split bundle of light is reflected toward the focusing screen 4 and is used for an observation of a photo-taking field of view. A portion of the other split bundle of light passing through the main mirror 14 is reflected by the half-reflection plane 17 at the object side and then impinges on the photoelectric conversion part 12 of the light measuring device, while the remaining portion of the bundle of light is reflected by the total-reflection plane 18 at the film plane 10 side and impinges into the photoelectric conversion part 13 of the focus detection device.
When the photoelectric conversion part 12 of the light measuring device is so positioned as mentioned above, a light measuring zone only within a viewfinder will not becomes awkwardly dark or the distribution of the light measuring sensitivity will not be influenced by the split image prism 4' of the focusing screen 4.
However, it is difficult to secure a housing space for the photoelectric conversion part 12 of the light measuring device and the photoelectric conversion part 13 of the focus detection device at the bottom part of the mirror box. Further the half-reflection plane 17 and the total-reflection plane 18 are formed at the beam splitting mirror 16. Therefore, there is a problem that the beam splitting mirror 16 becomes large.